US 20020159648 A1 shows the creation of a high-pass filtered image in a manner similar to that described in the previous paragraph.
It is a known fact that insufficient stability of the offset level for individual columns or rows over a focal plane array for IR detection, IR-FPA, can cause serious image disturbances in the form of pattern noise. In the case of low signal-to-noise ratios, the image is perceived as striped. In general, the offset level of the columns and rows deviates over time, which means that the resulting noise is spatio-temporal in nature. Usually, offset errors are calibrated over an IR-FPA, by having the array illuminated with a uniform radiator and then digitally deducting the offset level for the individual pixels, so-called image unevenness correction.
A problem in eliminating pattern noise according to the above-described process is that the pattern noise must be stable over time. This is seldom the case, however, as regards the offset level for columns and rows. Furthermore, an IR-FPA, apart from an offset correction, must also be corrected for errors in the amplification of individual pixels. This is done by digitally multiplying the amplification of the pixels by correction factors measured by illuminating an IR-FPA with uniform radiators at different temperatures. For this correction to be effective, it is also required that the pattern noise caused by offset errors is unchanged over time. Since this often is not the case for column and/or row noise, there is a risk that the offset error affects the measurement of the amplification corrections. This error will also appear in the form of a static column and/or row noise.